Look up books via Hardcover API

From what I can tell, Popfeed uses ISBN DB for looking up book information. Unfortunately, the ISBN DB API is a paid resource, so I don't believe it's practical to support for this plugin. That being said, we might be able to get all of the required book information -- including the cover image -- from the Hardcover API, which allows free access.

While this is technically possible due to the observed format of social.popfeed.feed.listItem records, it seems risky to support this feature without lexicon validation.